After two years of intense training with Dracule Mihawk, Zoro has grown much more powerful than before, but we don’t really know Zoro’s full power yet. Since the time-skip, Zoro has fought really weak enemies. We have only seen him fight against Monet, Pica and Killer. Monet didn’t even had haki, she’s too weak to fight against someone like Zoro. Pica is a haki user, but Zoro proved his own haki was stronger and cut Pica down with Sanzen Sekai.
The fight with Killer is the only important fight he had, and despite that there are too many variables. Zoro was poisoned and distracted, Killer was mentally and physically tortured and didn’t have his usual weapons, and Oda probably ended that fight fast because he expects the anime to add a bit more into it latter. This gives us a bad way of measuring Zoro’s power, but I think we all agree that Zoro can’t be stronger than Luffy.
It can be argued that up until Enies Lobby both were close in strength. But after that, and with the implementation of gears, haki and the fact that Luffy grows with every fight, Luffy is way stronger than Zoro right now. Luffy has had problems in his three main fights in the new world. Against Doflamingo, it was the time limit of gear fourth, with Cracker I don’t know what the result would be without Nami’s help, and the fight with Katakuri was his hardest battle.
In two out of these three fights he could have died, but despite that we can all agree that Luffy is right now first commander level, a level that he reached in Whole Cake island after fighting Katakuri. The important here is that Luffy had to fight 2 strong enemies (Doflamingo and Cracker) before he could defeat the strongest commander of an emperor, and even with that he had it difficult. So, it just feels wrong that, without any stepping stone, Zoro could jump up to where Luffy is right now.
There are many theories that Zoro will fight against King in the final battle. We don’t know King’s power either apart from his devil fruit and what he did with Big Mom Pirates. But again we can go with how Oda builds up villains: It doesn’t have much sense that King would be weaker than Katakuri, specially since as far as we’ve seen, Kaido is a guy that respects strength above everything. That’s why I don’t think that Zoro could fight King right now, even with his new sword: Enma. But I believe that Zoro could probably defeat Jack. However, it wouldn’t be easy for him.